Doped graphitic carbon nitrides, methods of making and uses of the same

ABSTRACT

Carbon-doped graphitic carbon nitride (g-C 3 N 4 ) compositions are synthesized from the chemical precursors melamine, cyanuric acid and barbituric acid. Phosphorus-doped g-C 3 N 4  compositions are synthesized from the chemical precursors melamine, cyanuric acid and etidronic acid. Carbon- and phosphorus-doped g-C 3 N 4  compositions, when in the presence of UV or visible light, can be used in water treatment systems to photocatalytically degrade persistent organic micropollutants such as pharmaceuticals and personal care products (PPCPs), endocrine disrupting compounds (EDCs), pesticides, and herbicides. Carbon- and phosphorus-doped g-C 3 N 4  compositions can also be applied to surfaces of household and public items to kill protozoa, eukaryotic parasites, algal pathogens, bacteria, fungi, prions, viruses, or other microorganisms, preventing the transfer thereof between users.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.62/295,624, filed Feb. 16, 2016, the contents of which are incorporatedby reference herein in its entirety.

FIELD OF TECHNOLOGY

The present application relates doped graphitic carbon nitride (g-C₃N₄)compositions and methods of making such compositions. The presentapplication also relates to the use of doped g-C₃N₄ compositions inwater treatment and antimicrobial applications.

BACKGROUND

A growing number of persistent organic micropollutants such aspharmaceuticals and personal care products (PPCPs), endocrine disruptingcompounds (EDCs), pesticides, and herbicides are frequently observed innatural and treated water. These contaminants are recalcitrant toconventional water and wastewater treatment and may pose risks to humanand ecological systems even at very low concentrations. Persistent andemerging waterborne, foodborne, and airborne pathogens can cause thespread of infectious diseases, and their control is important to protectthe public health. Photocatalysis is a promising advanced oxidationprocess (AOP) for the degradation or mineralization of persistentorganic micropollutants and the inactivation of pathogens because itactivates O₂ and/or H₂O at ambient conditions to generate reactiveoxygen species (ROS; e.g., .OH, O₂ ⁻./HO₂., H₂O₂, and ¹O₂) in situ.Photocatalysis also eliminates the hurdles in the storage, handling, andtransportation of oxidants or disinfectants, and potentially usesrenewable solar energy or indoor lighting for water purification andantimicrobial applications.

Recently, graphitic carbon nitride (g-C₃N₄) has emerged as avisible-light-responsive photocatalyst with tunable band gaps of 1.8-2.7eV that allow the harvesting of visible light up to 460-698 nm(potentially utilizing 13-49% of solar energy, though photocatalyticactivity may be reduced at a longer wavelength). g-C₃N₄ is made fromearth-abundant, inexpensive carbon and nitrogen containing precursors(e.g., urea and melamine), is biocompatible with no reported toxicity,is resistant to photo-corrosion, and remains chemically stable in harshenvironments.

Supramolecular preassembly of triazine precursors has become anattractive approach to tailor the properties and reactivity of g-C₃N₄.The supramolecular approach is more environmentally benign andsustainable compared to widely used hard-templating with nanosilicabecause no toxic or corrosive chemicals are involved (e.g., HF or NH₄HF₂for the post-removal of silica and pore generation). Cyanuric acid hasbeen applied with melamine because they can interact with each other byforming hydrogen bonds, producing a highly stable supramolecule as theprecursor of g-C₃N₄. Cyanuric acid is less thermally stable thanmelamine and decomposes into gases at an elevated temperature, which maycreate a porous structure of g-C₃N₄ with an increased surface area andcharge separation.

The molecular structure of g-C₃N₄ has also been altered with metaland/or nonmetal dopants, or nanoparticles to improve charge separationand visible-light utilization. Such methodologies are disadvantageous,however, because metal dopants or nanoparticles are more expensive(e.g., noble metal loading) and may leach or be deactivated in a complexenvironment.

BRIEF SUMMARY OF THE DRAWINGS

The accompanying figures illustrate embodiments and serve to explainprinciples of the disclosed embodiments. It is to be understood,however, that these figures are presented for purposes of illustrationonly, and not for defining limits of the present invention.

FIG. 1 is an illustration of the chemical structure of a portion of agraphitic carbon nitride (g-C₃N₄) sheet in accordance with variousembodiments of the present disclosure;

FIG. 2 is an illustration of a synthetic procedure for the formation ofsupramolecule-based, carbon-doped g-C₃N₄ (MCB_(x)) compositions inaccordance with various embodiments of the present disclosure;

FIG. 3 is an illustration of a synthetic procedure for the formation ofsupramolecule-based, phosphorus-doped g-C₃N₄ (MCE_(y)) compositions inaccordance with various embodiments of the present disclosure;

FIG. 4 is a block diagram of an exemplary water treatment systemaccording to various aspects of the present disclosure;

FIG. 5 is a graph displaying the spectral irradiance of a xenon lamp(λ>400 nm) for use in photocatalytic processes with g-C₃N₄ compositionsin accordance with various embodiments of the present disclosure;

FIG. 6 is an illustration of an undoped g-C₃N₄ sheet employed inelectronic structure calculations in accordance with various embodimentsof the present disclosure;

FIG. 7 is an illustration of designated sites for the doping of a g-C₃N₄sheet, the doping sites designated as N₁, N₂, N₃, C₁, C₂, and pore, inaccordance with various embodiments of the present disclosure;

FIG. 8 is an illustration of optimized geometries of undoped and dopedg-C₃N₄ compositions in accordance with various embodiments of thepresent disclosure;

FIG. 9 is a graphical display of phenol degradation over time usingMCB_(x) (x=0.01-1.9), under simulated visible light irradiation, inaccordance with various embodiments of the present disclosure;

FIG. 10 is a graphical display of phenol degradation over time usingMCE_(y) (y=0.01-0.5), under simulated visible light irradiation, inaccordance with various embodiments of the present disclosure;

FIG. 11 is a graphical display of atrazine degradation over time usingMCE_(y) (y=0.01-0.5), under simulated visible light irradiation, inaccordance with various embodiments of the present disclosure;

FIG. 12 is a graphical display comparing the photocatalytic degradationrate constants of phenol, atrazine, sulfamethoxazole and carbamazepineusing the following g-C₃N₄ compositions: 1) g-C₃N₄ synthesized from urea(U), 2) g-C₃N₄ synthesized from melamine (M), 3) g-C₃N₄ synthesized frommelamine and cyanuric acid (MC), and 4) g-C₃N₄ synthesized frommelamine, cyanuric acid, and barbituric acid (MCB_(0.07)), in accordancewith various embodiments of the present disclosure;

FIG. 13 is a graphical display of photocatalytic rate constants ofatrazine degradation using MCB_(0.07) under visible light irradiationover four cycles, in accordance with various embodiments of the presentdisclosure;

FIG. 14 is an attenuated total reflection-Fourier transform infraredspectroscopic (ATR-FTIR) spectra of freshly prepared and recoveredMCB_(0.07) before and after use in an atrazine degradation process inaccordance with various embodiments of the present disclosure;

FIG. 15 shows scanning electron microscopic (SEM) images of freshMCB_(0.07) (left) and MCB_(0.07) recovered after an atrazine degradationprocess (right) in accordance with various embodiments of the presentdisclosure;

FIG. 16 is a graphical display of the reactivity inhibition of phenoland atrazine degradation using U and MCB_(0.07) in the presence ofvarious scavengers (10 mM of tert-butyl alcohol as a .OH scavenger, 10mM of ammonium oxalate as a hole scavenger, and 10 mM of L-histidine asa ¹O₂ scavenger) in accordance with various embodiments of the presentdisclosure;

FIG. 17 is a graphical display of the reactivity inhibition of phenoldegradation using U in the presence of various scavengers (10 mM oftert-butyl alcohol as a .OH scavenger, 10 mM of ammonium oxalate as ahole scavenger, and 10 mM of L-histidine as a ¹O₂ scavenger, 2 U/mL ofsuperoxide dismutase as a O₂ ⁻. scavenger, and 200 U/mL of catalase as aH₂O₂ scavenger) in accordance with various embodiments of the presentdisclosure;

FIG. 18 shows graphical displays of atrazine degradation using (a)freshly prepared MCB_(0.07) suspension in simulated water samples, (b)24 hour-aged MCB_(0.07) suspension in simulated water samples, (c)freshly prepared MCB_(0.07) suspension in real water samples (raw or(partially) treated water and wastewater from plants), and (d) 24hour-aged MCB_(0.07) suspension in real water samples (raw or(partially) treated water and wastewater from plants) under simulatedvisible sunlight irradiation (xenon lamp, λ>400 nm) in accordance withvarious embodiments of the present disclosure;

FIG. 19 is a graphical display of E. coli disinfection over time usingg-C₃N₄ in accordance with various embodiments of the present disclosure;

FIG. 20 shows confocal laser scanning microscopic images of S.epidermidis biofilms developed on the surface of g-C₃N₄ coupons in thedark (left) and under visible light (right) in accordance with variousembodiments of the present disclosure;

FIG. 21 shows optical coherence tomographic (OCT) images of S.epidermidis biofilms developed on the surface of g-C₃N₄ coupons in thedark (left) and under visible light (right) in accordance with variousembodiments of the present disclosure;

FIG. 22 shows powder X-ray diffraction (XRD) patterns of differentg-C₃N₄ compositions (U, M, MC, and MCB_(0.07); a.u. represents arbitraryunits) in accordance with various embodiments of the present disclosure;

FIG. 23 shows ATR-FTIR spectra of different g-C₃N₄ compositions (U, M,MC, and MCB_(0.07)) in accordance with various embodiments of thepresent disclosure;

FIG. 24 shows an SEM image and a transmission electron microscopic (TEM)image (inset) of a g-C₃N₄ composition formed from urea (U), and furthershows a table (inset) providing the surface area, pore volume, and bulkcarbon to nitrogen (C/N) mass ratio of U in accordance with variousembodiments of the present disclosure;

FIG. 25 shows an SEM image and a TEM image (inset) of a g-C₃N₄composition formed from melamine (M), and further shows a table (inset)providing the surface area, pore volume, and bulk carbon to nitrogen(C/N) mass ratio of M in accordance with various embodiments of thepresent disclosure;

FIG. 26 shows an SEM image and a TEM image (inset) of a g-C₃N₄composition formed from melamine and cyanuric acid (MC), and furthershows a table (inset) providing the surface area, pore volume, and bulkcarbon to nitrogen (C/N) mass ratio of MC in accordance with variousembodiments of the present disclosure;

FIG. 27 shows an SEM image and a TEM image (inset) of a g-C₃N₄composition formed from melamine, cyanuric acid, and barbituric acid(MCB_(0.07)), and further shows a table (inset) providing the surfacearea, pore volume, and bulk carbon to nitrogen (C/N) mass ratio ofMCB_(0.07) in accordance with various embodiments of the presentdisclosure;

FIG. 28 shows SEM images of MCB_(0.07) prepared in ethanol (a) and water(b) in accordance with various embodiments of the present disclosure;

FIG. 29 is a graphical display of band gap properties of differentg-C₃N₄ compositions in accordance with various embodiments of thepresent disclosure;

FIG. 30 is a graphical display of optical absorbance of different g-C₃N₄composition suspensions (1 g/L of U, M, MC, and MCB_(0.07) in 1 mMphosphate buffer, pH 7.3) in accordance with various embodiments of thepresent disclosure; and

FIG. 31 is a graphical display of photoluminescence spectra of differentg-C₃N₄ compositions (U, M, MC, and MCB_(0.07); a.u. represents arbitraryunits) in accordance with various embodiments of the present disclosure.

DETAILED DESCRIPTION

Exemplary, non-limiting embodiments of the present application will nowbe described with references to the above-mentioned figures.Particularly, embodiments of the application comprise parts or methodsteps that are similar or identical to each other. These parts or methodsteps are thus denoted with similar or identical names or referencenumerals. Description of these relevant parts of method steps is herebyincorporated by reference, wherever relevant or appropriate.

As used throughout, ranges are used as shorthand for describing each andevery value that is within the range. Any value within the range can beselected as the terminus of the range. Unless otherwise specified, allpercentages and amounts expressed herein and elsewhere in thespecification should be understood to refer to percentages by weight.

For the purposes of this specification and appended claims, unlessotherwise indicated, all numbers expressing quantities, percentages orproportions, and other numerical values used in the specification andclaims, are to be understood as being modified in all instances by theterm “about.” The use of the term “about” applies to all numeric values,whether or not explicitly indicated. This term generally refers to arange of numbers that one of ordinary skill in the art would consider asa reasonable amount of deviation to the recited numeric values (i.e.,having the equivalent function or result). For example, this term can beconstrued as including a deviation of ±10 percent, alternatively ±5percent, and alternatively ±1 percent of the given numeric valueprovided such a deviation does not alter the end function or result ofthe value. Accordingly, unless indicated to the contrary, the numericalparameters set forth in this specification and attached claims areapproximations that can vary depending upon the desired propertiessought to be obtained by the present invention.

It is noted that, as used in this specification and the appended claims,the singular forms “a,” “an,” and “the,” include plural referencesunless expressly and unequivocally limited to one referent. As usedherein, the term “include” and its grammatical variants are intended tobe non-limiting, such that recitation of items in a list is not to theexclusion of other like items that can be substituted or added to thelisted items. For example, as used in this specification and thefollowing claims, the terms “comprise” (as well as forms, derivatives,or variations thereof, such as “comprising” and “comprises”), “include”(as well as forms, derivatives, or variations thereof, such as“including” and “includes”) and “has” (as well as forms, derivatives, orvariations thereof, such as “having” and “have”) are inclusive (i.e.,open-ended) and do not exclude additional elements or steps.Accordingly, these terms are intended to not only cover the recitedelement(s) or step(s), but may also include other elements or steps notexpressly recited. Furthermore, as used herein, the use of the terms “a”or “an” when used in conjunction with an element may mean “one,” but itis also consistent with the meaning of “one or more,” “at least one,”and “one or more than one.” Therefore, an element preceded by “a” or“an” does not, without more constraints, preclude the existence ofadditional identical elements.

For the purposes of this specification and appended claims, the term“coupled” refers to the linking or connection of two objects. Thecoupling can be permanent or reversible. The coupling can be direct orindirect. An indirect coupling includes connecting two objects throughone or more intermediary objects. The term “substantially” refers to anelement essentially conforming to the particular dimension, shape orother word that substantially modifies, such that the component need notbe exact. For example, substantially cylindrical means that the objectresembles a cylinder, but can have one or more deviations from a truecylinder.

FIG. 1 is an illustration of the chemical structure of graphitic carbonnitride (g-C₃N₄). g-C₃N₄ as shown in FIG. 1 can be synthesized by, forexample, heating a powder of any of urea, melamine, dicyandiamide, orthiourea at elevated temperatures for a predetermined period of time,followed by cooling to room temperature. g-C₃N₄ is in the form of atwo-dimensional sheet and characterized by tri-s-triazine (C₆N₇) units,as elementary building blocks, and a pore bounded by threetri-s-triazine units. In some instances, g-C₃N₄ can also be formed froma mixture of melamine and cyanuric acid.

According to various aspect of the present disclosure, carbon (C)-dopedsupramolecule-based g-C₃N₄ (MCB_(x)) compositions, described herein, canbe used to degrade and/or decompose persistent organic micropollutantssuch as PPCPs, EDCs, pesticides, and herbicides that are frequentlyobserved in natural and treated water. Generally, MCB_(x) compositionscan be made by formation of a supramolecular complex from predeterminedamounts of the precursors melamine, cyanuric acid, and barbituric acid,followed by thermal polycondensation, as illustrated in FIG. 2. Tosynthesize MCB_(x) compositions, the following general methodology canbe utilized. First, predetermined amounts of melamine, cyanuric acid,and barbituric acid are dispersed in a solvent to form a suspension. Thesolvent can be, for example, an alcohol such as methanol or ethanol,water, chlorinated solvents such as chloroform or methylene chloride,acetone, an ether such as diethyl or diphenyl ether, and aldehyde, aketone, any combination thereof, or any other suitable solvent orsolvent system. The suspension can then be stirred and agitated for aperiod of time suitable for the suspension to become substantiallyhomogeneous. Agitation can be, for example, shaking, ultrasonication ina bath or using a probe, or any other suitable agitation process knownto one of ordinary skill in the art. The substantially homogeneoussuspension can then be subjected to elevated temperature to remove thesolvent and to form the supramolecular complex. The dried supramolecularcomplex can then be placed in a chemically and thermally inertcontainer, such as an alumina or ceramic crucible, and heated to atemperature sufficient to form the final MCB_(x) composition. Thetemperature used for formation of the final MCB_(x) composition canrange from about 450° C. to about 600° C., alternatively about 500° C.to about 600° C., and alternatively about 550° C.

MCB_(x) compositions can be synthesized from a precursor mixturecomprising about 50 wt % melamine, about 2.5-49.75 wt % cyanuric acid,and about 0.25-47.5 wt % barbituric acid. For example, preparation ofMCB_(x) on a four gram scale can utilize 2 grams of melamine, 0.1 to1.99 grams of cyanuric acid and 0.01 to 1.9 grams of barbituric acid. Insome instances, on a four gram scale, 0.01 to 0.1 grams of barbituricacid is used. As described herein, a MCB_(x) composition formed from,for example, 2 grams of melamine, 1.93 grams of cyanuric acid, and 0.07grams of barbituric acid is named as “MCB_(0.07).” One of ordinary skillin the art can readily appreciate that more or less MCB_(x) can beproduced by scaling the above described values.

The addition of barbituric acid to melamine and cyanuric acid can leadto C-doped g-C₃N₄ compositions via substitution of one or more nitrogenatoms within a tri-s-triazine unit, or via substitution of nitrogenatoms linking tri-s-triazine units together, with a carbon atom (FIG.2). MCB_(x) compositions according to the present disclosure aresubstantially mesoporous (i.e. having pores with diameters ranging fromabout 2 to 50 nm) with surface areas ranging from about 70 m²/g to about100 m²/g. MCB_(x) compositions according to the present disclosure mayalso have pore volumes ranging from about 0.5 cm³/g to about 0.8 cm³/g.MCB_(x) compositions according to the present disclosure may also havecarbon to nitrogen (C/N) mass ratios ranging from about 0.60 to about0.70.

According to various aspects of the present disclosure, phosphorus(P)-doped supramolecule-based g-C₃N₄ (MCE_(y)) compositions, describedherein, can be used to degrade and/or decompose persistent organicmicropollutants such as PPCPs, EDCs, pesticides, and herbicides that arefrequently observed in natural and treated water. Generally, MCE_(y)compositions can be made by formation of a supramolecular complex frompredetermined amounts of the precursors melamine, cyanuric acid, andetidronic acid, followed by thermal polycondensation, as illustrated inFIG. 3. To synthesize MCE_(y) compositions, the following generalmethodology can be utilized. First, predetermined amounts of melamine,cyanuric acid, and etidronic acid are dispersed in a solvent to form asuspension. The solvent can be, for example, an alcohol such as methanolor ethanol, water, chlorinated solvents such as chloroform or methylenechloride, acetone, an ether such as diethyl or diphenyl ether, andaldehyde, a ketone, any combination thereof, or any other suitablesolvent or solvent system. The suspension can then be stirred andagitated for a period of time suitable for the suspension to becomesubstantially homogeneous. Agitation can be, for example, shaking,ultrasonication in a bath or using a probe, or any other suitableagitation process known to one of ordinary skill in the art. Thesubstantially homogeneous suspension can then be subjected to elevatedtemperature to remove the solvent and to form the supramolecularcomplex. The dried supramolecular complex can then be placed in achemically and thermally inert container, such as an alumina or ceramiccrucible, and heated to a temperature sufficient to form the finalMCE_(y) composition. The temperature used for formation of the finalMCE_(y) composition can range from about 450° C. to about 600° C.,alternatively about 500° C. to about 600° C., and alternatively about550° C.

MCE_(y) compositions can be synthesized from a precursor mixturecomprising about 50 wt % melamine, about 37.5-49.75 wt % cyanuric acid,and about 0.25-12.5 wt % etidronic acid. For example, preparation ofMCE_(y) on a four gram scale can utilize 2 grams of melamine, 1.5 to1.99 grams of cyanuric acid and 0.01 to 0.5 grams of etidronic acid. Insome instances, on a four gram scale, 0.01 to 0.1 grams of etidronicacid is used. As described herein, a MCE_(y) composition formed from,for example, 2 grams of melamine, 1.97 grams of cyanuric acid, and 0.03grams of etidronic acid is named as “MCE_(0.03).” One of ordinary skillin the art can readily appreciate that more or less MCE_(y) can beproduced by scaling the above described values.

The addition of etidronic acid to melamine and cyanuric acid can lead toP-doped g-C₃N₄ compositions via substitution of one or more nitrogen orcarbon atoms within a tri-s-triazine unit, and/or via substitution ofnitrogen atoms linking tri-s-triazine units together with a phosphorusatom. In some instances, the addition of etidronic acid to melamine andcyanuric acid can lead to P-doped g-C₃N₄ compositions wherein phosphorusatoms chelate in a bidentate fashion to nitrogen atoms oriented towardthe pore (FIG. 3). MCE_(y) compositions according to the presentdisclosure are substantially mesoporous (i.e. having pores withdiameters ranging from about 2 to 50 nm) with surface areas ranging fromabout 70 m²/g to about 100 m²/g. MCE_(y) compositions according to thepresent disclosure may also have pore volumes ranging from about 0.5cm³/g to about 0.8 cm³/g.

In some instances, computational quantum mechanical modelling methodssuch as, but not limited to density functional theory (DFT),time-dependent density functional theory (TDDFT), chemical or moleculardynamics, semi-empirical and empirical methods, and ab initio methodscan be used to rationally design MCB_(x) and MCE_(y) compositions tohave thermodynamically stable structures, suitable charge separation,and suitable energy levels of conduction and valence bands to promotephotocatalytic oxidation.

In some instances, the presently described MCB_(x) and MCE_(y)compositions can be used in the presence of ultraviolet (UV) or visiblelight as a photocatalyst in water treatment applications. Specifically,MCB_(x) and MCE_(y) compositions can be used to degrade persistentorganic micropollutants such as PPCPs, EDCs, pesticides, and herbicideswhich are frequently observed in natural and treated water. In general,the target pollutants can be anything that degrades or decomposes in thepresence of ROS (e.g., —OH, O₂ ⁻./HO₂., H₂O₂, and ¹O₂) in situ. In thepresent disclosure, phenol, atrazine, sulfamethoxazole, andcarbamazepine are discussed as target pollutants/contaminants. One ofordinary skill in the art, however, can readily appreciate that MCB_(x)and MCE_(y) compositions can be used to degrade or decompose targetpollutants/contaminants having chemical structures similar to suchcompounds.

In some instances, MCB_(x) and MCE_(y) compositions can be used todegrade aliphatic or aromatic hydrocarbons. In other instances, MCB_(x)and MCE_(y) compositions can be used to degrade aliphatic or aromaticalcohols. In yet other instances, MCB_(x) and MCE_(y) compositions canbe used to degrade substituted or unsubstituted triazines. In yet otherinstances, MCB_(x) and MCE_(y) compositions can be used to degradesubstituted or unsubstituted oxazoles or oxazodines. In yet otherinstances, MCB_(x) and MCE_(y) compositions can be used to degradesubstituted or unsubstituted pyrroles, pyridines, purines, andpyrimidines. In yet other instances, MCB_(x) and MCE_(y) compositionscan be used to degrade substituted or unsubstituted azepines. In yetother instances, MCB_(x) and MCE_(y) compositions can be used to degradealdehydes, ketones, esters and carboxylic acids. In yet other instances,MCB_(x) and MCE_(y) compositions can be used to degrade amines, imines,amides, and sulfonamides. While specific examples of compounds which canbe degraded by MCB_(x) and MCE_(y) compositions are provided above, oneof ordinary skill in the art can readily appreciate that the MCB_(x) andMCE_(y) compositions disclosed herein can be used to degrade anycompounds which are reactive to ROS (e.g., .OH, O₂ ⁻./HO₂., H₂O₂, and¹O₂).

In some instances, where a MCB_(x) or MCE_(y) composition is used as aphotocatalyst in water treatment applications, the MCB_(x) or MCE_(y)composition can be formed into any one of a bulk powder, a porous brickor sponge-like material, a wafer, a film, or any other suitable shape.The shaped MCB_(x) or MCE_(y) composition can be placed in a cartridge,piping, or similar containment means. The cartridge or similarcontainment means can have a fluid inlet for entry of contaminated wateror fluid therein and a fluid outlet for the water or fluid to exit afterdegradation of the contaminant(s) by the MCB_(x) or MCE_(y) composition.The cartridge, piping or similar containment means should be transparentor substantially transparent to allow for the passage or UV or visiblelight to pass therethrough. In some instances, thin films, sheets wafersor coupons can be formed by subjecting a bulk powder of a MCB_(x) orMCE_(y) composition, or a combination of MCB_(x) and MCE_(y)compositions, to high pressure, such as a hydraulic or pneumatic press.In other instances, a bulk powder of a MCB_(x) or MCE_(y) composition,or a combination of MCB_(x) and MCE_(y) compositions, can be formed intoa shaped porous brick or sponge-like material using a tablet press orsimilar device. The tablet press or similar device can have any suitabledimensions to make shaped porous bricks or sponge-like materials havingdimensions for small to industrial scale water treatment applications.

FIG. 4 is a block diagram of an exemplary water treatment systemaccording to various aspects of the present disclosure. The watertreatment system 400 includes a water purification assembly 410 and acontaminated water source 420. The water purification assembly 410includes a top surface 411, a transparent plate 413, a contaminatedwater stream 414 having a contaminated water inlet (not shown) and aconcentrated contaminated water outlet (not shown), a membrane 415including a MCB_(x) composition and/or a MCE_(y) composition, andpurified water stream 416 having a purified water outlet (not shown).The top surface 411 includes an aperture 412 for the transmission of UVor visible light therethrough to irradiate the MCB_(x) compositionand/or MCE_(y) composition-containing membrane 415 for the production ofROS. Contaminated water is transmitted from the contaminated watersource 420 to the contaminated water stream 414, is purified by themembrane 415, and then enters the purified water stream 416. Apressurized gas source 430 is fluidically coupled with the contaminatedwater source 420 to assist in transmission of the contaminated waterfrom the contaminated water source 420 to the contaminated water stream414. The purified water outlet of the purified water stream 416 isfluidically coupled with an inlet (not shown) of a purified watercontainer 440. The remaining contaminated water, now concentrated, istransmitted from the contaminated water stream 414, through theconcentrated contaminated water outlet, to an inlet (not shown) of awater bath cooler 470 with the aid of a pump 460. The contaminated watermay then re-enter the contaminated water source 420 from the water bathcooler 470, for further purification. A pressure vent valve 450 may bedisposed between the contaminated water stream 414 and the pump 460.While the water treatment system 400 of FIG. 4 includes numerouscomponents, one of ordinary skill in the art can readily appreciate thatother components can be added to the water treatment system 400 withoutimparting from the scope of the present disclosure.

In some instances, the presently described MCB_(x) and MCE_(y)compositions can be used in the presence of UV or visible light as aphotocatalyst in antimicrobial applications. Specifically, MCB_(x) andMCE_(y) compositions can be used to inactivate protozoa, eukaryoticparasites, algal pathogens, bacteria, fungi, prions, viruses, or othermicroorganisms which are frequently observed in water, air, food, or onany surfaces. In general, MCB_(x) and MCE_(y) compositions can be usedagainst any protozoa, eukaryotic parasites, algal pathogens, bacteria,fungi, prions, viruses, or other microorganism that inactivates,degrades or decomposes in the presence of ROS (e.g., .OH, O₂ ⁻/HO₂.,H₂O₂, and ¹O₂) in situ.

To inactivate protozoa, eukaryotic parasites, algal pathogens, bacteria,fungi, prions, viruses or other microorganisms, compositions comprisingMCB_(x) and/or MCE_(y) compositions can be applied to any surface basedon desired use. For instance, compositions comprising MCB_(x) and/orMCE_(y) compositions can be applied to surfaces, including portionsgripped by a hand of a user, of kitchen counter-tops, cutting boards,cabinet, drawer or appliance handles or knobs to degrade potentiallydangerous bacteria or viruses which may be transferred thereon duringthe preparation of meals or foodstuffs. In other instances, compositionscomprising MCB_(x) and/or MCE_(y) compositions can be applied to thesurface of food processing machines and food packages. In otherinstances, compositions comprising MCB_(x) and/or MCE_(y) compositionscan be applied to the surfaces of furniture such as, for example desks,tables, lighting fixtures (for example, lamps), and so on to degradecommunicable viruses or bacteria that may be transferred thereon byusers, preventing the spread of such communicable viruses or bacteria toother users. In yet other instances, compositions comprising MCB_(x)and/or MCE_(y) compositions can be applied to the surfaces of light orpower switches. In yet other instances, the compositions comprisingMCB_(x) and/or MCE_(y) compositions can be applied to surface of, orimpregnated in, woven textiles such as, but not limited to, yarns,fabrics, articles of clothing, medical or laboratory garments, drapery,and so on. In yet other instances, compositions comprising MCB_(x)and/or MCE_(y) compositions can be applied to any surface of interest inhospitals, doctors' or dentists' offices, rehabilitation facilities,hospice care facilities, etc. where the chances of the spread ofcommunicable viruses or bacteria is especially high.

In some instances, MCB_(x) and MCE_(y) compositions may be used todegrade or inactivate bacteria such as, but not limited to, E. coli,staphylococcal species such as S. epidermidis, S. haemolyticus and S.aureus, Legionella species such as Legionella pneumophila, Vibriocholera, Campylobacter, Brucella, Clostridium perfringens, Listeria,Salmonella, and Shigella.

In some instances, MCB_(x) and MCE_(y) compositions may be used todegrade or inactivate viruses such as, but not limited to, norovirus,rotavirus, adenovirus, and strains of influenza.

In some instances, MCB_(x) and MCE_(y) compositions may be used todegrade or inactivate protozoa such as, but not limited to,Cryptosporidium parvum, Giardia lamblia, and Naegleria fowleri.

In some instances, MCB_(x) and/or MCE_(y) compositions can be applied tosurfaces with the aid of an adhesive. In such instances, an adhesivelayer can first be disposed on a surface of an article and a layer of aMCB_(x) composition, a MCE_(y) composition, or a combination thereof canbe disposed on the adhesive layer. In some instances, the MCB_(x)composition, the MCE_(y) composition, or combination thereof can bedisposed upon the adhesive layer as a bulk powder using a powder coatingmethod, a spray coating method or any other suitable powder applicationmethod known to one of ordinary skill the art. In other instances, aMCB_(x) composition, a MCE_(y) composition, or a combination thereof canfirst be formed into a film or sheet as described above and the film orsheet can be applied on the adhesive layer using a roll-to-roll coatingtechnique, a lamination technique such as heated roll lamination or coldroll lamination, or any other suitable film or sheet applicationtechnique known to one of ordinary skill in the art. Adhesives suitablefor application of a MCB_(x) composition, a MCE_(y) composition, or acombination thereof to a surface of an article can include, but is notlimited to epoxies, polyurethanes, polychloroprene, polyamides,polyimides, polyacrylates, natural rubbers, shellac, polyvinyl compounds(such as polyvinyl acetate, alcohol, butyral, chloride, etc.),polyethylene, polypropylene, or any other suitable adhesive known to oneof ordinary skill in the art.

In other instances, MCB_(x) and/or MCE_(y) compositions can be dispersedin a solvent to form a MCB_(x) and/or MCE_(y) containing suspension. Thesuspension can then be applied to a surface of an article andsubsequently dried. Such methods can be, but are not limited to,dip-coating spin-coating, slit-coating, drop-casting, doctor blading,and inkjet printing, or any other suitable suspension method known toone of ordinary skill in the art.

In some instances, other additives, such as binders, rheology modifiers,and the like, can be incorporated into the MCB_(x) or MCE_(y) solutionto modify its coating properties. Any additives added, however, shouldbe substantially transparent to allow for the passage of UV or visiblelight therethrough to prevent deleterious effects to the photocatalyticefficiency of the MCB_(x) or MCE_(y) compositions. Furthermore, anyadhesive or additive(s) used should be non-reactive or substantiallynon-reactive in the presence of ROS to prevent the degradation of theadhesive or additive(s) and loss of the MCB_(x) or MCE_(y) compositionsfrom the surface by, for example, delamination.

EXAMPLES

The following examples are intended to be illustrative only, and are notintended to be, nor should they be construed as, limiting in any way ofthe scope of the present disclosure.

I. Materials and Methods

Reagents

All chemicals were at least reagent grade and used as received. Thesynthesis of g-C₃N₄ involved urea (Sigma-Aldrich, 98%), melamine (AcrosOrganic, 99+%), cyanuric acid (Sigma-Aldrich, 98%), barbituric acid(Sigma-Aldrich, 99%), etidronic acid monohydrate (Sigma-Aldrich, 95+%),and ethanol (Sigma-Aldrich, 99.5%). A buffer solution of 1 mM potassiumphosphate monobasic (Fisher Scientific, 99.3%) was adjusted to pH 7.3and used in photocatalytic experiments. Phenol (Sigma-Aldrich, 99+%),atrazine (Sigma-Aldrich, 98.8%), carbamazepine (Sigma-Aldrich, 98+%),and sulfamethoxazole (Sigma-Aldrich, 99+%) were used as probecontaminants in photocatalytic studies. Potassium nitrate (FisherScientific, 99.9%), humic acid sodium salt (Sigma-Aldrich), magnesiumchloride (Fisher Scientific, 99.4%), calcium chloride dihydrate (FisherScientific, 74.3%), and sodium hydrosulfide hydrate (Sigma-Aldrich, NaHS60+%) were selected as representative natural water constituents orfoulants for photocatalytic reactions. Ammonium oxalate (Sigma-Aldrich,99+%), tert-butyl alcohol (Sigma-Aldrich, 99+%), superoxide dismutase(from bovine erythrocytes, Sigma-Aldrich, >3,000 units/mg of protein),catalase (from bovine liver, lyophilized powder, Sigma-Aldrich,2,000-5,000 units/mg of protein), and L-histidine (Sigma-Aldrich, 99+%)were selected as scavengers for oxidative species in photocatalyticreactions. The eluents for high performance liquid chromatographic(HPLC) analyses of pollutants consisted of sodium acetate(Sigma-Aldrich, anhydrous), potassium phosphate monobasic (FisherScientific, 99.3%), acetonitrile (ACN) (Fisher Scientific, 99.9%), andmethanol (Sigma-Aldrich, 99.9%). All solutions were prepared inultrapure water (Millipore, Milli-Q, 18.2 MW cm).

Density Functional Theory (DFT) Simulations

Electronic structure calculations were conducted using DFT asimplemented in the CP2K suite. An initial cell relaxation was performedwith the Perdew-Burke-Ernzerhof (PBE) density functional, amoderately-sized double-zeta valence polarized (DZVP) basis set, andGoedecker-Teter-Hutter (GTH) pseudopotentials (grid cutoff of 300 Ry),in a hybrid Gaussian-plane wave (GPW) framework. This optimized cellgeometry was then used to define subsequent relaxations for undoped anddoped materials via the range-separated, hybrid Heyd-Scuseria-Ernzerhof06 (HSE06) functional. Energy shifts due to solvation were quantified byperforming a reference calculation on undoped g-C₃N₄ using an implicitmodel.

All structures with nitrogen site substitutions were assumed to beuncharged, while interstitially doped systems and those with dopants atcarbon sites were assigned a charge of +1. The relative stability ofeach dopant family was determined by comparing geometries optimizedusing undoped g-C₃N₄ cell parameters, excluding the nitrogen to carbonsubstitutions which used the average of optimized cell parameters forthat series of dopants.

Synthesis of g-C₃N₄, Carbon Doped g-C₃N₄, and Phosphorus-Doped g-C₃N₄

The conventional g-C₃N₄ samples were synthesized from urea (U) ormelamine (M). 10 g of urea or melamine powder was put into an aluminacrucible with a cover (not sealed), heated at a rate of 2.3° C./min andmaintained at 550° C. for 4 h in a muffle furnace, and then cooled downnaturally.

The supramolecule-based g-C₃N₄ samples without non-metal doping weresynthesized from melamine and cyanuric acid (MC, mass ratio 1:1).

Carbon-doped, supramolecule-based g-C₃N₄ samples (MCB_(x)), weresynthesized from melamine, cyanuric acid, and barbituric acid withdifferent mass ratios (i.e., 2 g of melamine, (2-x) g of cyanuric acid,and x g of barbituric acid; x=0-1.9).

Phosphorus-doped, supramolecule-based g-C₃N₄ samples (MCE_(y)), weresynthesized from melamine, cyanuric acid, and etidronic acid withdifferent mass ratios (i.e., 2 g of melamine, (2-y) g of cyanuric acid,and y g of etidronic acid; y=0-0.5).

The synthetic procedure of the MC, MCB_(x), and MCE_(y) samples was asfollows. Precursors were first dispersed in 40 mL of ethanol to formsuspension. The suspension was next stirred at ambient temperature for 3h, followed by sonication (Elamsonic P, 37 kHz, 100 W) at roomtemperature for an additional 3 h. The suspension was then dried on ahot plate at 70° C. until no obvious liquid was found, and whitesupramolecular aggregates were formed. Finally, the dried supramolecularaggregates were put into an alumina crucible with a cover (not sealed),heated at a rate of 2.3° C./min and maintained at 550° C. for 4 h in amuffle furnace, and cooled down naturally.

Characterization of g-C₃N₄ Carbon-Doped g-C₃N₄ and Phosphorus Dopedg-C₃N₄

The crystal phase of g-C₃N₄ was determined by X-ray powder diffraction(XRD) analyses on a Rigaku Miniflex+ diffractometer with Cu Kαradiation.

Sample morphologies were characterized with a scanning electronmicroscope (SEM, JEOL 6700F) and a transmission electron microscope(TEM, Philips CM300 FEG). For SEM, g-C₃N₄ samples dispersed in ethanolwere cast and dried on a sample stub, and SEM micrographs were collectedat 10 kV with a secondary electron detector. For TEM, g-C₃N₄ samplesdispersed in ethanol were also cast and dried on a Cu grid with a carbonsupport, and TEM images were collected at 300 kV. The elementaldistribution of MCE_(y) was characterized by SEM-energy dispersivespectroscopy (EDS) (SEM, TESCAN MIRA3 FEI; EDS, TEAM Octane SSD) at anaccelerating voltage of 20 kV.

Surface properties were investigated by X-Ray photoelectron spectroscopy(XPS). A PHI 5600 system was used for analysis under UHV conditions(pressure <10⁻⁸ Torr). An Mg Kα source (1253.6 eV) was utilized, andejected photoelectrons were measured with a hemispherical energyanalyzer operating at 58.7 eV constant pass-energy. Peak positions werereferenced to C1s, 284.5 eV, and CasaXPS was used to determine chemicalcomposition and atomic concentrations at the surface (up to ca. 10 nm).

Attenuated total reflectance-Fourier transform infrared spectroscopic(ATR-FTIR) spectra were collected using a Nicolet 6700 spectrometer from4000-525 cm⁻¹ (32 scans at 4 cm⁻¹ resolution). Spectra were peaknormalized by the C—N asymmetric stretch at 1231 cm⁻¹, a region presentin every sample tested with relative invariant spectral intensity.

Brunauer-Emmitt-Teller (BET) surface area and porosity analyses wereperformed by N₂ adsorption/desorption using a Micromeritics Tri Star3000. Isotherm adsorption data for P₀/P was recorded from 0.06-0.989.The volume of micropores was determined by t-plot analysis and thevolume of mesopores was determined by BJH analysis. Samples weredegassed at 140° C. for 12 h under dynamic vacuum (10⁻³ Torr) prior toanalysis.

Bulk carbon, hydrogen, and nitrogen were analyzed on Model CE 440 CHNAnalyzer. The capsule containing g-C₃N₄ samples was injected into a hightemperature (1000° C.) furnace and combusted in pure O₂ under staticconditions. To ensure the complete combustion, a dynamic burst of O₂ wasadded at the end of the combustion period. The resulting combustionproduct contained CO₂, H₂O, and N₂/NO_(x), and it next passed over Cu toscrub excess O₂ and reduce NO_(x) to N₂. After scrubbing, the gasesentered a mixing volume chamber to ensure a homogenous mixture atconstant temperature and pressure, and were detected by high-precisionthermal conductivity detectors. An H₂O trap and a CO₂ trap were usedbetween the detectors, and the differential signal before and after thetrap was proportional to the H₂O concentration and CO₂ concentration.Finally, N₂ was measured against a He reference.

Optical absorbance spectra of the photocatalysts and their band gap weredetermined by a Thermo Scientific Evolution 300 UV-vis spectrophotometerwith a Praying Mantis diffuse reflectance infrared Fourier transformspectroscopy (DRIFTS) accessory. The absorbance of g-C₃N₄ samplesirradiated by light over a wavelength scan from 200 to 800 nm wasmeasured, and the value was then converted into reflectance by equation1:A=log(1/R)  (1)where A is the measured absorbance and R is the reflectance. Thefunction α, representative of optical absorption, which is equivalent toadsorption coefficient divided by scattering coefficient for g-C₃N₄, wascalculated from the Kubelka-Munk formula:α=(1−R)²/2R  (2)The band gap was obtained by extrapolating the linear portion of(αhv)^(1/2) versus photon-energy plots at (αhv)^(1/2)=0, h is Planck'sconstant (4.14×10⁻⁵ eV·s), v is the frequency of photons which can beobtained by dividing c, the speed of light (3.0×10⁸ m/s) by λ, thephoton's wavelength.

Photoluminescence (PL) spectra were obtained from a home-made apparatusbased on a Thermo Nicolet Nexus 670 rapid scan FTIR spectrometer. The PLspectra of the samples were obtained using a 354 nm diode pumped solidstate laser from Teem Photonics as excitation source. The intensity onthe samples was 30 μW focused to a spot of 100 μm in diameter. Theemitted fluorescence was focused to a 300 mm path length monochromatorand collected by a thermoelectrically-cooled charge-coupled device (CCD)camera, both from Princeton Instruments. A long wavelength pass filtercutting at 420 nm was used to block the laser light into themonochromator.

Zeta potential and hydrodynamic diameter of g-C₃N₄ aqueous suspensionswere determined using folded capillary cells (DTS 1061, Malvern) on aZetasizer Nano ZS instrument (Malvern, ZEN3600). The temperature wasmaintained at 25° C., and the scattering angle was 173° from theincident laser beam.

Ultraviolet-visible (UV-vis) spectra of g-C₃N₄ aqueous suspension wasobtained on a Thermo Scientific Evolution 201PC spectrophotometer. Theabsorption spectra of the catalyst suspension were measured in theregion of 200 to 800 nm with at a resolution of 2 nm. The sample cellwas a quartz cuvette (1 cm by 1 cm). The concentration of the preparedcatalysts was 1 g/L in the aqueous solution, and the suspension wasdispersed using a bath sonicator (100 W) for 10 min.

Light Source Description and Characterization

A xenon arc lamp (1000 W) was selected as a light source in the presentExamples. A long-pass optical filter with a cut-off wavelength of 400 nmwas used to simulate visible light irradiation at earth's surface.Spectral irradiance, photon fluence, and optical powder density of thexenon lamp were recorded by a spectroradiometer (AvaSpec ULS2048L). Thespectral irradiance of the xenon lamp (λ>400 nm) was plotted in FIG. 5,and the photon fluence and the optical powder density were 601 (μmol ofphotons)/m² s and 16.7 mW/cm², respectively. Applicant notes that g-C₃N₄with a band gap of 2.72 eV only absorbs photons with a wavelength up toabout 460 nm.

Photocatalytic Activity Testing in a Phosphate Buffer

First, 15 mg of g-C₃N₄ was mixed with ultrapure water and dispersed viasonication (Elamsonic P, 37 kHz, 100 W) for 15 minutes. The dispersedg-C₃N₄ suspension and a probe pollutant (i.e., phenol, atrazine,carbamazepine, or sulfamethoxazole) were added to a phosphate buffersolution in a jacketed reactor, giving a total reaction volume of 15 mL,the g-C₃N₄ loading of 1 g/L, the phosphate buffer concentration of 1 mM(pH 7.3), and the pollutant concentration of 100 μM. The reactor had adiameter of 4 cm and starting depth of suspension of 1.5 cm. The surfaceof the suspension was centered 20 cm below the xenon lamp, which had abeam width of 6 cm. The reaction temperature was maintained at 25° C.The suspension was stirred for 20 minutes in the dark in order to ensuresuspension homogeneity, and a sample of 300 μL was next taken toquantify the initial pollutant concentration. Pollutant adsorption ontog-C₃N₄ was negligible, based on the comparison of the measured initialconcentration with the concentration in control reactors in absence ofg-C₃N₄. The reactor was then irradiated under the xenon lamp (λ>400 nm,typically for 60 minutes), during which aqueous aliquots were withdrawnperiodically. The aliquots were next centrifuged at 13,000 rpm for 1hour in the dark to allow the particles to settle. The supernatant wastransferred to a 1.5 mL amber autosampler vial for subsequent analysisvia HPLC with photodiode array detection (Shimadzu LC-20AT ProminenceHPLC-DAD). For atrazine degradation over multiple photocatalytic cycles,g-C₃N₄ was harvested by centrifugation after each cycle, rinsed withultrapure water, and reused for the next run. The photocatalyst usedafter four cycles was characterized by SEM-EDS and ATR-FTIR.

Photocatalytic Activity Testing in a Phosphate Buffer

To explore the influence of water chemistries on photocatalyticperformance, particularly water matrices representative of watertreatment systems, the reactivity of MCB_(0.07) for atrazine degradationwas explored in simulated water samples (1 mM phosphate buffer solutionwith the presence of 5 mM NaHCO₃, 50 mM CaCl₂, 50 mM MgCl₂, 5 mg/L humicacid sodium salt, 5 mg/L NaNO₃ as N, or 0.1 mM NaHS; pH 7.3 for allsolutions except for NaHCO₃ solution with pH 7.5). Ca²⁺ and Mg²⁺ wereselected to represent the hardness species in water, NO₃ ⁻, humic acid,and HS⁻ were selected as representative scavengers for oxidative speciesgenerated from photocatalytic reactions, and HCO₃ ⁻ was selected torepresent alkalinity species that could also react with the oxidativespecies. Atrazine degradation was also conducted in real water samplescollected from a water treatment plant (hereinafter “GWP”) and awastewater reclamation facility (hereinafter “BRWRF”). Samples werecollected from the raw water of GWP (GWP-1), the final effluent from GWP(GWP-4), the effluent after membrane bioreactor treatment in BRWRF(BRWRF-1), and the final effluent from BRWRF (BRWRF-3), and filteredthrough a 0.45 μm polyvinylidene fluoride (PVDF) membrane beforephotocatalytic reactions. For the exploration of long-term effect ofcomplex water matrices for photocatalytic activity, MCB_(0.07) was alsoadded to the water samples and magnetically stirred in the dark for 24hours to pre-foul the photocatalyst before the photocatalytic reactions.Atrazine was added to the reactor 20 minutes before irradiation toensure suspension homogeneity, and again, there was no evidence ofadsorption in any system. pH and alkalinity of these real water samplesare listed in Table 1.

TABLE 1 GWP-1 GWP-4 BRWRF-1 BRWRF-3 pH 6.7 7.2 7.5 7.0 Alkalinity 56 6187 N/A (mg/L as CaCO₃)HPLC Analysis

The Shimadzu LC-20AT Prominence HPLC System was equipped with a ShimadzuC18 column (4.6×50 mm, 1.8 μm particle size).

The HPLC analysis method for phenol was based on previous work (Arana,J., et al.; Photocatalytic degradation of phenolic compounds with newTiO₂ catalysts. Appl. Catal., B 2010, 100 (1-2), 346-354) and employed amobile phase of 1 mM sodium acetate: ACN (65:35) at pH 3, a flow rate of0.75 mL/min, an injection volume of 20 μL, and a 254 nm detectionwavelength.

The HPLC analysis method for atrazine was based on previous work(Newcombe, D. A., et al.; Bioremediation of atrazine-contaminated soilby repeated applications of atrazine-degrading bacteria. Appl.Microbiol. Biotechnol. 1999, 51 (6), 877-882) and employed a mobilephase of ultrapure water: ACN (50:50), a flow rate of 1 mL/min, aninjection volume of 100 μL, and a 223 nm detection wavelength.

The HPLC analysis method for carbamazepine was based on previous work(Borisover, M., et al.; Enhancement effect of water associated withnatural organic matter (NOM) on organic compound-NOM interactions: Acase study with carbamazepine. Chemosphere 2011, 82 (10), 1454-1460) andhad a mobile phase of ultrapure water: ACN (55:45), a flow rate of 1mL/min, an injection volume of 25 μL, and a 213 nm detection wavelength.

The HPLC analysis method for sulfamethoxazole was based on previous work(Nalbandian, M. J., et al.; Tailored synthesis of photoactive TiO2nanofibers and Au/TiO2 nanofiber composites: Structure and reactivityoptimization for water treatment applications. Environ. Sci. Technol.2015, 49 (3), 1654-1663) and had a mobile phase of 5 mM potassiumphosphate monobasic (pH adjusted to 5.0): methanol (70:30), a flow rateof 1 mL/min, an injection volume of 20 μL, and a 268 nm detectionwavelength.

II. Results and Discussion

DFT Simulations

Preliminary DFT studies were performed on an undoped, HSE06-optimized(3×3) g-C₃N₄ supercell (FIG. 6), revealing a planar geometry and abandgap of 2.75 eV (Table 2, below). The conduction and valence bandedges (−1.03 and 1.71 eV), measured with respect to the absolutepotential of the standard hydrogen electrode (SHE; 4.44 eV), likewiselie near known experimentally determined values of −0.83 and 1.83 eV.Due to the accuracy of DFT simulations, carbon and phosphorus doping atthree inequivalent nitrogen sites (N₁, N₂, and N₃), phosphorussubstitution at inequivalent carbon atoms (C₁ and C₂), and theincorporation of phosphorus within the g-C₃N₄ pore (i.e., interstitialdoping), all with one dopant introduced to one doping site ofHSE06-optimized (3×3) g-C₃N₄ supercell were explored. The dopedmaterials are labeled as A→B doped g-C₃N₄ for further discussion (A isthe doping site of N₁, N₂, N₃, C₁, C₂, and Pore, and B is the dopingelement of C or P). The N₁, N₂, N₃, C₁, C₂, and Pore doping sites areillustrated in FIG. 7.

To determine the thermodynamic ability of these materials to generateROS (e.g., .OH, O₂ ⁻/HO₂., H₂O₂, and ¹O₂) and holes, their valence bandmaximum (VBM) and conduction band minimum (CBM) was quantified inconduction with an implicit solvation model, represented by the highestoccupied molecular orbital (HOMO) and lowest unoccupied molecularorbital (LUMO) energies, respectively. Table 2 provides calculatedvalues of the band gap, the CBM and the VBM of g-C₃N₄ doped with carbonor phosphorus at various locations, in conformity with the descriptionof FIG. 6, based on DFT simulations. The CBM and VBM values are versus astandard hydrogen electrode (SHE). Reported redox potentials for O₂/O₂⁻., H₂O₂/.OH, O₂/H₂O₂, O₂ ⁻./H₂O₂, and .OH/H₂O are −0.16, 0.32, 0.70,0.94 and 2.33 V vs. SHE, respectively. According to the DFT simulations,N₁→C, N₃→C, N₁→P, N₂→P, and N₃→P doped g-C₃N₄ can produce O₂ ⁻. viaone-electron reduction of O₂ because their CBM is more negative than theredox potential of O₂/O₂ ⁻. (E₀(O₂/O₂ ⁻.)=−0.16 V vs SHE). All dopedmaterials are able to produce H₂O₂ and —OH from the one-electronreduction of O₂ ⁻. and H₂O₂, respectively (E₀(O₂ ⁻./H₂O₂)=0.94 V andE₀(H₂O₂/.OH)=0.32 V vs SHE). However, O₂ reduction for O₂ ⁻. formationis the first step for ROS production on the conduction band (i.e.,sequential reduction of O₂→O₂ ⁻.→H₂O₂→.OH), and those doped g-C₃N₄ thatcannot reduce O₂ may not produce sufficient ROS and thus result in lowphotocatalytic activity for contaminant degradation. Multielectronreduction could facilitate ROS production on the conduction band becauseit is more thermodynamically favorable than one-electron reduction(E₀(O₂/H₂O₂)=0.70 V vs SHE), but the loading of co-catalysts (e.g., Pt,Au, and Cu(II)) may be required.

TABLE 2 Composition (Doping site) Band Gap (eV) CBM (V) VBM (V) Undopedg-C₃N₄ 2.75 −1.03 1.71 N₁ → C doped g-C₃N₄ 2.58 −1.01 1.57 N₂ → C dopedg-C₃N₄ 2.08 0.05 2.13 N₃ → C doped g-C₃N₄ 2.94 −0.67 2.28 N₁ → P dopedg-C₃N₄ 2.06 −0.71 1.36 N₂ → P doped g-C₃N₄ 1.69 −0.20 1.49 N₃ → P dopedg-C₃N₄ 2.90 −0.70 2.20 C₁ → P doped g-C₃N₄ 1.93 0.12 2.05 C₂ → P dopedg-C₃N₄ 1.86 0.19 2.05 Pore → P doped g-C₃N₄ 1.14 −0.06 1.08

FIG. 8 is an illustration of the optimized geometry of undoped and dopedforms of g-C₃N₄. As illustrated, both N₁→C and N₃→C doped g-C₃N₄ exhibita planar geometry similar to that of undoped g-C₃N₄. Conversely, N₂→Cdoped g-C₃N₄ is characterized by a significant out-of-plane deformation.The N₁₋₃→P geometries are collectively more distorted, each exhibitingsome degree of undulation due to the bulky phosphorus substituent, andare presumably less thermodynamically stable than their carbon-dopedcounterparts due to strain. Following these observations, the holedistribution in the valence band (HOMO) and electron distribution of theconduction band (LUMO) of these materials were determined. While theHOMO of undoped g-C₃N₄ is relatively delocalized, the LUMO exhibits astaggered distribution on alternating trimers. The same trend isobserved for the LUMO of carbon-doped systems, though N₂→C doped g-C₃N₄exhibits a higher degree of localization due to a pronounced “tenting”of the g-C₃N₄ sheet. The HOMO of carbon-doped g-C₃N₄ is markedlydifferent from the undoped material and is distributed around the dopantsite. While the orbital structure will undergo remodeling subsequent toexciton formation and charge migration, it is reasonable to speculatethat the impurity-localized valence electrons provide the primary sitesfor localized hole formation within the valence band of doped g-C₃N₄.

Photocatalytic Performance of C- and P-Doped g-C₃N₄

The effect of nonmetal doping on the photocatalytic performance ofg-C₃N₄ was evaluated by testing MCB_(x) or MCE_(y) for phenol and/oratrazine degradation under simulated visible sunlight irradiation (λ>400nm, xenon lamp) and comparing the activity with MC.

FIG. 9 is a graphical representation of the photocatalytic activity forphenol degradation of MCB_(x) compositions synthesized with varyingamounts of barbituric acid. As shown in FIG. 10, the synthesized MCB_(x)exhibited significantly enhanced photocatalytic activity, especially atthe low barbituric acid content (i.e., x=0.01-0.07). MCB_(0.07) had thehighest reaction rate constant, and its reactivity was enhanced by2.8-fold compared to that of MC (0.8±0.08 vs 0.3±0.02 m²/(mol ofphotons). The photocatalytic activity of MCB_(x) significantly decreasedat higher barbituric acid mass loadings (x>0.1), likely due to excessivecarbon doping promoting charge recombination.

FIG. 10 is a graphical representation of the photocatalytic activity forphenol degradation of MCE_(y) compositions synthesized with varyingamounts of etidronic acid. As can be seen, only MCE_(0.03) andMCE_(0.05) performed marginally better than the undoped MC composition.

FIG. 11 is a graphical representation of the photocatalytic activity foratrazine degradation of MCE_(y) compositions synthesized with varyingamounts of etidronic acid. As can be seen, reactivity increase wasobserved for atrazine degradation on MCE_(0.01) that had the lowestphosphorus doping level 0.6±0.1 and 0.3±0.02 m²/(mol of photons) forMCE_(0.01) and MC, respectively). A slight activity increase was alsoobserved for MCE_(0.03) as compared to MC.

The experimental results are in agreement with DFT simulation resultswith carbon-doped g-C₃N₄ showing an enhanced activity compared to thatof phosphorus-doped g-C₃N₄. The carbon-doped supramolecule-based g-C₃N₄with an optimum photocatalytic activity (MCB_(0.07)), undopedsupramolecule-based g-C₃N₄ (MC), and conventional g-C₃N₄ (U and M) wereselected for further material characterization and water treatmentapplications.

Relationship Between Photocatalytic Activity and g-C₃N₄ Properties.

FIG. 12 shows photocatalytic degradation kinetics of phenol andpersistent organic micropollutants on g-C₃N₄ compositions in thephosphate buffer under simulated visible sunlight irradiation (λ>400 nm,xenon lamp). MCB_(0.07) showed the highest photocatalytic activity foratrazine, sulfamethoxazole, and carbamazepine compared to that of otherg-C₃N₄ samples, and its reactivity was on par with U for phenoldegradation. M had the lowest reactivity compared to that of itscounterpart g-C₃N₄ samples. Of the four probe contaminants, atrazine isthe most photocatalytically degradable on g-C₃N₄ samples (except for U),and carbamazepine is the most persistent in photocatalytic reactions.

Atrazine degradation was also explored on MCB_(0.07) over four repeatedcycles, and no reactivity loss or morphological/chemical property changeof the photocatalyst was observed. For each of the four cycles, anMCB_(0.07) loading of 1 g/L, an initial atrazine concentration of 100μm, a 1 mM phosphate buffer (pH 7.3), and xenon lamp irradiation atλ>400 nm was used. FIG. 13 is a graphical display of photocatalytic rateconstants of atrazine degradation on MCB_(0.07) under visible lightirradiation over the four cycles (error bars represent 95% confidenceintervals. FIG. 14 is an ATR-FTIR spectra comparing fresh MCB_(0.07) andMCB_(0.07) recovered after atrazine degradation. FIG. 15 shows SEMimages of fresh MCB_(0.07) (left) and MCB_(0.07) recovered afteratrazine degradation (right).

The measured photocatalytic activity for contaminant degradation couldbe influenced by mass transfer and contaminant adsorption on g-C₃N₄.Nonetheless, as will be discussed below, mass transfer limitation waseliminated in the reaction, and no contaminant adsorption was measuredin the dark experiments. In addition, all g-C₃N₄ samples showed asimilar negative ζ-potential (i.e., −28.5 to −37.8 mV at pH 7.3), andthey were not expected to interact strongly with neutral contaminants inthe reaction solution at pH 7.3 (pKa's of phenol, atrazine,sulfamethoxazole, and carbamazepine are 10.0, 1.7, 1.4/5.8, 2.3/13.9,respectively).

The enhancement of photocatalytic activity could be determined by thefollowing key photocatalyst properties: (i) an increased surface area,(ii) improved light harvesting, and (iii) promoted charge separation.Photocatalysis is a surface-mediated reaction, and the increased surfacearea provides more active sites for the contaminants to reside andreact. The improvement of light harvesting and charge separation enablesthe generation of more electrons and holes for photocatalytic reactions.MCB_(0.07) exhibited the highest surface area, and its reactivityoutperformed M and MC for the degradation of all probe contaminants. Thecharge separation of MCB_(0.07) and MC was significantly promotedcompared to M, and M showed the lowest reactivity for contaminantdegradation. The low carbon doping of MCB_(0.07) did not change its bandgap, but midgap states possibly increased visible light absorption andphotocatalytic activity. The results indicate that the enhanced surfacearea, visible light absorption, and charge separation of MCB_(0.07)synergistically contribute to improving the photocatalytic activity.

Selective Contaminant Degradation on g-C₃N₄.

It is worth noting that g-C₃N₄ is selective for contaminant degradation.MCB_(0.07) is 1.8 times more reactive for the degradation of atrazinethan phenol; however, U is 3.9 times more reactive for the degradationof phenol than atrazine. Similar selective degradation ofsulfamethoxazole was also observed in comparison with phenol oratrazine. To understand the mechanism of selective contaminant removal,the significance of the ROS and hole for the photocatalytic degradationof phenol and atrazine was explored. Scavenger tests were conducted toquench the reactions between contaminants and a specific oxidativespecies, and the inhibition of reactivity was used to evaluate thecontribution of the specific oxidative species to photocatalyticdegradation. The results, as shown in FIG. 16, indicated that theaddition of tert-butyl alcohol (10 mM), a .OH scavenger, reduced thereactivity for phenol degradation by 60% on U compared to 33% onMCB_(0.07). The .OH scavenger test also showed 55% of reactivityinhibition for atrazine degradation on U, in contrast to no inhibitionfor atrazine degradation on MCB_(0.07). However, the addition ofammonium oxalate (10 mM), a hole scavenger, significantly inhibited thereactivity for phenol and atrazine degradation on both U and MCB_(0.07)from 74 to 86%. The addition of L-histidine (10 mM), a ¹O₂ scavenger,reduced the reactivity of phenol degradation by 76% on U compared to 55%on MCB_(0.07) and fully inhibited the reactivity for atrazinedegradation on both U and MCB_(0.07). For these experiments, aphotocatalyst (U or MCB_(0.07)) loading of 1 g/L, an initial atrazineconcentration of 100 μm, a 1 mM phosphate buffer (pH 7.3), and xenonlamp irradiation at λ>400 nm was used.

With being bound to any particular theory, it is believed that theproduction of oxidative species on g-C₃N₄ is structurally sensitive;different g-C₃N₄ compositions generate different types and amounts ofoxidative species. Phenol is known to be more reactive with .OH thanatrazine with .OH, and the second-order rate constant of .OH-phenol is5.9-fold higher than that of .OH-atrazine (1.4×10¹⁰ vs 2.4×10⁹ s⁻¹ M⁻¹).U may generate more .OH than MCB_(0.07), and quenching .OH had morepronounced inhibition for phenol degradation. MCB_(0.07) may produce asignificant amount of other ROS, e.g., O₂ ⁻./HO₂. and ¹O₂, to facilitateatrazine degradation. Though .OH is generally more reactive(second-order reaction rate constant between ROS and organics are10⁶-10¹⁰, 10³-10¹⁰, or <10⁸ M⁻¹ s⁻¹ for .OH, ¹O₂, or O₂ ⁻./HO₂.,respectively), its steady-state concentration is orders of magnitudelower in water compared to that of other ROS. The other ROS, especially¹O₂, may be a leading contributor to atrazine degradation; quenching .OHhad a negligible effect on atrazine degradation on MCB_(0.07), butquenching ¹O₂ completely deactivated photocatalytic atrazinedegradation. In contrast, U could produce .OH as one dominant ROS, andhence atrazine degradation on U is slower, and quenching .OH reduces thereactivity significantly. The hole and ¹O₂ are the most dominantoxidative species for contaminant degradation, regardless of g-C₃N₄properties. O₂ ⁻. and H₂O₂ were also identified in the photocatalyticdegradation of phenol on U, and their contribution to reaction kineticswas also critical (FIG. 17). In FIG. 17, the reactivity inhibition (%)was calculated by dividing the reactivity difference without and withthe scavenger by the reactivity without the scavenger. For theseexperiments, a photocatalyst (U or MCB_(0.07)) loading of 1 g/L, aninitial atrazine concentration of 100 μm, a 1 mM phosphate buffer (pH7.3), and Xenon lamp irradiation at λ>400 nm was used.

Performance Comparison in Matrixes Representative of Water andWastewater Treatment.

For all experiments conducted in the simulated and real water samplesand with both fresh and aged MCB_(0.07), little to no inhibition wasobserved in atrazine degradation, as shown in FIG. 18. Graphical display(a) of FIG. 18 shows atrazine degradation by a freshly preparedMCB_(0.07) suspension in simulated water samples. Graphical display (b)of FIG. 18 shows atrazine degradation by a 24-hour aged MCB_(0.07)suspension in simulated water samples. Graphical display (c) of FIG. 18shows atrazine degradation by a freshly prepared MCB_(0.07) suspensionin real water samples. Graphical display (d) of FIG. 18 shows atrazinedegradation by a 24-hour aged MCB_(0.07) suspension in real watersamples. In the experiments, simulated visible sunlight irradiation(xenon lamp, λ>400 nm) was used. The initial concentration of atrazinewas 100 μM in simulated water samples and 20 μM in real water samples.Atrazine degradation was faster with the lower initial concentration of20 μM. Control experiments were conducted under the same experimentalconditions in a phosphate buffer (pH 7.3, 1 mM) prepared from ultrapurewater. The foulants or natural water constituents were amended into thephosphate buffer for simulated water tests. No phosphate buffer was usedfor the real water tests. Photocatalyst loading was 1 g/L. The excellentphotocatalytic performance of g-C₃N₄ in complex water matrixes may beattributed to the large surface area, high porosity, the polymericnature of the material without metal loading, and the selective ROSproduction.

Increased surface area and porosity may afford sufficient sites tointeract with contaminants even when foulants or natural waterconstituents are at a high concentration (i.e., mM of foulants orconstituents vs μM of contaminants). Noble metals used in photocatalysis(e.g., Au) are reactive and susceptible to fouling, and the polymericnature of g-C₃N₄ with the presence of only carbon and nitrogen mitigatesthe likelihood of photocatalyst deactivation. In addition, thehydrophilic nature of g-C₃N₄ reduces the adsorption of organic orbiological foulants, and the tri-s-triazine unit in g-C₃N₄ is notexpected to bind strongly to the carboxylate group of NOM, in contrastto TiO₂.

The selective production of ROS may also contribute to the minimalinhibition of photocatalytic activity in complex water matrices. —OH isthe most powerful, nonselective radical generated in most AOPs and isattractive for environmental applications because it can degrade mostorganic contaminants at near diffusion-limited rates. However, thenonselective destruction of contaminants can be easily compromised whennon-target chemicals are present, including NOM and other backgroundconstituents present in raw natural water or (partially) treated waterand wastewater. In the presently described system, MCB_(0.07) maygenerate other ROS predominantly rather than .OH, such as ¹O₂ and O₂⁻./HO₂., which are more selective for contaminant degradation. Forexample, ¹O₂ selectively oxidizes electron-rich moieties of contaminantswith the presence of NOM, and O₂ ⁻./HO₂. facilitates the degradation ofhalogenated compounds because of the synergy of reductive dehalogenationand the oxidation of carbon backbones (O₂ ⁻./HO₂. can act as both areductant and an oxidant).

The exploration of the structural sensitivity is significant for theapplication of g-C₃N₄ in contaminant degradation because the synthesisand properties of g-C₃N₄ compositions can be tailored to selectivelydegrade a certain group of highly toxic, low concentration contaminantswith minimized inhibition from co-contaminants, natural waterconstituents, or foulants. It may also be useful for designingphotocatalysts for selective oxidation and conversion of contaminants tovalue-added products (e.g., benzene oxidation to phenol). Furtherstudies may be helpful to quantitatively analyze the production ofoxidative species and identify key g-C₃N₄ properties for the selectivegeneration of oxidative species and degradation of contaminants. Factorsbeyond the generation of oxidative species, such as the affinity ofg-C₃N₄ to contaminants, may also be important for the selectivity incontaminant degradation. The affinity between the photocatalyst andcontaminant may be tailored for selective reactions.

Influence of Mass Transfer Processes on Observed Reaction Rates

The following calculations were performed to investigate the potentialeffects of aqueous/solid and intraparticle mass transfer processes onthe observed reaction rates. The results indicate that mass transfer wasnot expected to limit the observed reaction rates in the presentlydisclosed systems.

Aqueous/Solid Mass Transfer Limitations

Here, the lower limit for the aqueous/solid mass transfer rate constantfor atrazine degradation on MCB_(0.07) (k_(aq/s)a) was estimated andcompared to the measured reaction rate constant. Atrazine degradation onMCB_(0.07) was selected because it showed the fastest degradationkinetics compared to the other photocatalytic reactions of the presentapplication. Criteria for the evaluation of potential influence ofaqueous/solid mass transfer limitations on the observed rate constantsare 1) No aqueous/solid mass transfer limitations expected if calculatedk_(aq/s)a is much greater than the largest measured k_(obs, MCB) value;and 2) Significant potential for aqueous/solid mass transfer limitationsis expected if k_(aq/s)a is close to or less than the measuredk_(obs, MCB) value.

The slip velocity method is used to calculate the mass transfercoefficient (k_(aq/s)) for particles traveling at the slip velocity(u_(t)) relative to the suspending liquid. Because the particle size ofMCB_(0.07) was much smaller than 1 mm, Stokes' law is assumed to beapplicable and the particle's slip velocity is calculated by:

$\begin{matrix}{u_{t} = \frac{{gd}_{p}^{2}\left( {\rho_{p} - \rho} \right)}{18\mu}} & (3)\end{matrix}$where g is the gravity constant, d_(p) and ρ_(p) are the hydrodynamicdiameter and density of the photocatalyst particle, respectively, ρ isthe density of water, and μ is the absolute viscosity of water.

The aqueous/solid mass transfer coefficient is then estimated by thefollowing expression:

$\begin{matrix}{k_{{aq}/s} = {{\frac{D_{mol}}{d_{p}}{Sh}} = {\frac{D_{mol}}{d_{p}}\left( {2 + {0.6\mspace{20mu}{Re}^{0.5}{Sc}^{0.33}}} \right)}}} & (4)\end{matrix}$where D_(mol) is the molecular diffusion coefficient of the reactingsolute (atrazine), Sh is the Sherwood number, Re is the modifiedReynold's number, and Sc is the Schmitt number. The last two parametersare calculated by the following expressions:

$\begin{matrix}{{Re} = \frac{d_{p}u_{t}}{v_{H_{2}O}}} & (5) \\{{Sc} = \frac{v_{H_{2}O}}{D_{mol}}} & (6)\end{matrix}$where ν_(H2O) is the kinematic viscosity of water. The moleculardiffusion coefficient for atrazine in water is calculated using:

$\begin{matrix}{D_{mol} = \frac{13.26 \times 10^{- 5}}{{\mu^{1.14}(v)}^{0.589}}} & (7)\end{matrix}$where D_(mol) is in the unit of cm²/s, μ is in the unit of g m⁻¹ s⁻¹,and v′ is the molar volume of atrazine, which is 250.6 cm³/mol,calculated using the LeBas method. Plugging the results into equation 5,using μ=1.002 g/(m s) (at 20° C.), then D_(mol)=5.11×10⁻¹⁰ m²/s. Thevalues and other physical constants summarized in Table 3 are then usedin equations 3-6 to determine that k_(aq/s)=4.30×10⁻⁴ m/s.

TABLE 3 Constant Value D_(mol) of atrazine 5.11 × 10⁻¹⁰ m²/s Gravityconstant, g 9.81 m/s² Photocatalyst particle 2.39 × 10⁻⁶ m, determineddiameter, d_(p) by dynamic light scattering Kinematic viscosity 1.003 ×10⁻⁶ m²/s (at 20° C.) of water, ν_(H2O) Particle Density, ρ_(p) 1.34 ×10⁶ g/m³ Water density, ρ 9.98 × 10⁵ g/m³ (at 20° C.) Absolute viscosityof water, μ 1.002 g/(m s) (at 20° C.)

To calculate the mass transfer rate constant, the mass transfercoefficient should be multiplied by the geometric surface area of thecatalyst per volume of solution, a:

$\begin{matrix}{a = {\frac{{total}\mspace{14mu}{surface}\mspace{14mu}{area}}{{total}\mspace{14mu}{volume}} = {\frac{{SA}_{p} \times M}{\rho_{p} \times V_{p}} \times \frac{1}{V_{R}}}}} & (8)\end{matrix}$where SA_(p) is the geometric surface area of one MCB_(0.07) aggregate,which is assumed to have a spherical structure; M is the mass ofphotocatalyst in the reactor; V_(p) is the volume of one MCB_(0.07)aggregate; and VR is the volume of reaction suspension. For the fastestreaction measured, M=0.015 g and VR=0.015 L, so a=1871 m⁻¹ andk_(aq/s)a=0.8 s⁻¹. This value is about 3 orders-of-magnitude larger thanthe measured k_(obs,MCB) value (8.0×10⁻⁴ s⁻¹). Thus the results indicatethe aqueous/solid mass transfer is expected to have negligiblelimitation on the measured reaction rates.

Intraparticle Mass Transfer Limitations

The potential of intraparticle mass transfer resistance is alsoinvestigate by using the following criteria: 1) no resistance to porediffusion if (k_(obs)L²)/D_(e)<1, and 2) significant resistance to porediffusion if (k_(obs)L²)/D_(e)>1, where L is characteristic diffusionpath length for the photocatalyst and D_(e) is the effective diffusivityof the reacting solute. L and D_(e) are estimated using the followingequations:

$\begin{matrix}{L = {\frac{1}{6}d_{p}}} & (9) \\{D_{e} = \frac{D_{mol}\theta}{\tau}} & (10)\end{matrix}$where θ is the porosity of the photocatalyst particle (typically rangingfrom 0.2-0.7) and τ is the tortuosity factor (typically ranging from2-10). The most conservative values from each range are used to maximizethe possibility that the criterion (k_(obs)L²)/D_(e)>1 is met (i.e., wechose the smallest θ (0.2) and largest τ (10) in order to obtain thesmallest possible D_(e) value), and, in turn, the largest possible valuefor equation 9 is L=3.99×10⁻⁷ m and the largest possible value forequation 10 is D_(e)=1.02×10⁻¹¹ m²/s.

The largest observed reaction rate constant was 8.0×10's⁻¹ for atrazinedegradation on MCB_(0.07). This rate constant and the values of L andD_(e) calculated above are then used to test the two criteria outlinedabove to find a (k_(obs)L²)/D_(e) value of 1.25×10⁻⁵ (<<1). Because the(k_(obs)L²)/D_(e) value is several orders-of-magnitude less than one,intraparticle mass transfer resistance is negligible on the time scaleover which the photocatalytic degradation was observed the presentlydescribed system.

Photocatalytic Performance of MCB_(0.07) g-C₃N₄ in AntimicrobialApplications

Disinfection Experiments of Planktonic E. coli.

To generate a standard curve, E. coli was cultured in Luria-Bertanibroth (LB) at 37° C. with mixing (120 rpm) and was harvested during itslate-exponential phase by centrifugation. The E. coli was thenresuspended with phosphate-buffered saline (PBS) and then the suspensionwas centrifuged. The supernatant was discarded. This process wasrepeated for three time. After being washed with PBS, the E. coli wasresuspended in PBS to form a stock bacterial suspension. Next, the stocksuspension was diluted with PBS to form a series of concentrations (10%,20%, . . . , 100% of the original concentration). The OD₆₀₀ values ofbacterial solutions with different concentrations were measuredrespectively. Five concentrations were selected to conduct platecounting. Bacterial solutions were diluted in series. The relationshipbetween solution concentration and colony-forming units (CFU) wererecorded.

To test the antimicrobial activity of g-C₃N₄, 25 ml of a bacterialsuspension (E. coli in PBS, 1×10⁷ CFU/mL) was mixed with 0.001 g ofMCB_(0.07) g-C₃N₄ powder in a sterile glass beaker. The beaker wasplaced under a white light emitting diode (LED) lamp (7 W) for bacterialinactivation. The distance between the surface of the bacterialsuspension and the LED lamp was maintained at 15 cm. Bacterialsuspension samples were withdrawn from the beaker with pipette everyhalf an hour. The samples were duplicated, diluted in series with thePBS buffer, and plate counting was conducted to determine bacterialviability (FIG. 19). As can be seen in FIG. B, after two hours, thenumber of culturable E. coli decreased from 1×10⁷ to about 4×10⁴ CFU/mL,equating to a 99.6% reduction.

Biofilm Development Experiments.

S. epidermidis was cultured in tryptic soy broth (TSB) at 37° C. withmixing (120 rpm). S. epidermidis was harvested during itslate-exponential phase by centrifugation and diluted in the PBS bufferto prepare a bacterial suspension (OD₆₀₀=0.5). MCB_(0.07) g-C₃N₄ powderwas then hydraulically pressed to form thin coupons. The MCB_(0.07)g-C₃N₄ coupons were placed into a sterile six-well plate, and completelysubmerged by the bacterial suspension (2 ml for each coupon in eachwell) to form a system. The system was first incubated at 37° C. for 24h without light and mixing to ensure effective bacterial attachment onthe MCB_(0.07) g-C₃N₄ coupon surface. Then, the suspension was evacuatedby aspiration, and the coupons were gently rinsed with PBS three timesto remove loosely attached cells. Next, the coupons were transferred toa new sterile six-well plate, and 2 ml of 10-fold diluted TSB was addedto submerge the coupons. The system was incubated at 37° C. with amixing rate of 80 rpm under LED irradiation and in the dark (controlexperiment). TSB was replenished every 24 hours for the system and thesystem was incubated for 3 days. At the end of the experiments, theMCB_(0.07) g-C₃N₄ coupons were taken out and gently rinsed with the PBSbuffer three times. The Filmtracer LIVE/DEAD Biofilm Viability Kit wasused to stain the biofilms on the coupons, and a confocal microscope wasutilized to image the biofilms. The confocal microscopy results areshown in FIG. 20. A thick biofilm with a majority of live cells wasdeveloped on the g-C₃N₄ coupon in the dark, however, negligible bacteriasurvived on the g-C₃N₄ under LED irradiation.

Optical Coherence Tomography (OCT) Analysis.

S. epidermidis was cultured in tryptic soy broth (TSB) at 37° C. withmixing (120 rpm). S. epidermidis was then harvested during itslate-exponential phase by centrifugation and diluted in the PBS bufferto prepare a bacterial suspension (OD₆₀₀=0.5). MCB_(0.07) g-C₃N₄ powderwas then hydraulically pressed to form thin coupons. The MCB_(0.07)g-C₃N₄ coupons were placed into a sterile six-well plate, and completelysubmerged by the bacterial suspension (2 ml for each coupon in eachwell) to form a system. The system was first incubated at 37° C. for 24h without light and mixing to ensure effective bacterial attachment oncoupon surface. Then, the suspension was evacuated by aspiration, andthe coupons were gently rinsed with PBS three times to remove looselyattached cells. Next, the coupons were transferred to a new sterilesix-well plate, and 2 ml of 10-fold diluted TSB was added to submergethe coupons. The system was incubated at 37° C. with a mixing rate of 80rpm under LED irradiation and in the dark (control experiment). TSB wasreplenished every 24 hours for the system and the system was incubatedfor 2 days. The coupons were analyzed by optical coherence tomography(OCT) in the six-well plates directly. FIG. 21 shows OCT images ofbiofilms developed on the surface of MCB_(0.07) g-C₃N₄ coupons in thedark (left) and under visible light (right). A biofilm with a thicknessof 50-100 μm were developed on the g-C₃N₄ coupon in the dark, however,no biofilm was observed on the MCB_(0.07) g-C₃N₄ under LED irradiation.

Characterization of g-C₃N₄ Compositions

FIG. 22 is a graphical representation of the XRD patterns of g-C₃N₄compositions formed from urea (U), melamine (M), melamine and cyanuricacid (MC), and melamine, cyanuric acid, and barbituric acid(MCB_(0.07)), respectively. As can be seen, all g-C₃N₄ samples had asimilar XRD pattern that could be ascribed to a typical graphite-likestructure and the introduction of cyanuric acid and/or barbituric acidmay promote polymer-like growth and more defects in g-C₃N₄.Specifically, the peak 13° corresponding to in-plane ordering oftri-s-triazine units is less pronounced for supramolecule-based g-C₃N₄(that is, MC and MCB_(0.07)) than M, indicating that more defects may begenerated in the growth of g-C₃N₄ with the addition of cyanuric acidand/or barbituric acid. The other peak around 27.6°, representative of(002) interlayer stacking, became broader for supramolecule-based g-C₃N₄and U, suggesting their crystallinities are lower than that of M. Theresults may indicate that polymer-like growth was dominated in U, MC,and MCB_(0.07) compared to the growth of a graphitic structure in M.

ATR-FTIR analysis of each photocatalyst (FIG. 23) showed absorptionbands characteristic of g-C₃N₄: six bands in the 1200-1650 cm⁻¹ regionfor stretching modes of C—N heterocycles and one band at 805 cm⁻¹ fortriazine.

FIG. 24 is an SEM image and a TEM image (inset) of a g-C₃N₄ compositionformed from urea (U). FIG. 26 further provides a table (inset) providingthe surface area, pore volume, and bulk carbon to nitrogen (C/N) massratio of the U composition. The g-C₃N₄ composition formed from urea hasa porous structure, possibly due to a large amount of gas production inthe thermal polycondensation of urea (e.g., NH₃ and H₂O).

FIG. 25 is an SEM image and a TEM image (inset) of a g-C₃N₄ compositionformed from melamine (M). FIG. 27 further provides a table (inset)providing the surface area, pore volume, and bulk carbon to nitrogen(C/N) mass ratio of the M composition. A typical condensed, layeredstructure of the g-C₃N₄ composition formed from melamine was observedwith the presence of fewer pores than when formed from urea, likely dueto a lesser extent of structural reorganization and reduced gasemission.

FIG. 26 is an SEM image and a TEM image (inset) of a g-C₃N₄ compositionformed from melamine and cyanuric acid (MC). FIG. 28 further provides atable (inset) providing the surface area, pore volume, and bulk carbonto nitrogen (C/N) mass ratio of the MC composition. The addition ofcyanuric acid created a loose, porous structure, as expected from theself-templating of cyanuric acid with limited thermal stability.

FIG. 27 is an SEM image and a TEM image (inset) of a g-C₃N₄ compositionformed from melamine, cyanuric acid and barbituric acid (MCB_(0.07)).FIG. 29 further provides a table (inset) providing the surface area,pore volume, and bulk carbon to nitrogen (C/N) mass ratio of theMCB_(0.07) composition. The addition of cyanuric acid created a loose,porous structure, as expected from the self-templating of cyanuric acidwith limited thermal stability. Further introduction of barbituric acidfor MCB_(0.07) synthesis did not lead to any notable change in thesample morphology, possibly because of the low loading of barbituricacid (i.e., 0.07 g).

The surface area and porosity of g-C₃N₄ samples were characterized byliquid N₂ adsorption, and the adsorption isotherms and pore sizedistribution indicated that U, MC, and MCB_(0.07) compositions mainlycontained mesopores. The M composition had the lowest BET surface areaand pore volume in all g-C₃N₄ samples, as supported by the SEM and TEMresults. The BET surface area and pore volume of the MC compositionincreased by 8.0- and 6.6-fold compared to those of the M compositionand were also higher than those of the U composition. The MCB_(0.07) hadthe highest surface area and pore volume compared to other compositions.Also, the C/N mass ratio increases from 0.59 (U and M compositions) to0.60 (the MC composition) to 0.61 (the MCB_(0.07) composition).

FIG. 28 shows SEM images of MCB_(0.07) prepared in ethanol (a) and water(b). As shown, preparation in ethanol leads to MCB_(0.07) having apancake-like structure while preparation in water leads to MCB_(0.07)having an irregular rod-/particle-like structure.

Optical properties of the g-C₃N₄ compositions, including band gap andoptical absorption, were characterized to evaluate its capability ofharvesting and utilizing visible photons. g-C₃N₄ is an indirect band gapphotocatalyst, and all g-C₃N₄ compositions had a similar band gap of2.72 eV (harvesting photons with λ≤ca. 460 nm, FIG. 29), which is inagreement with the presently disclosed DFT simulation results forundoped g-C₃N₄. The addition of cyanuric acid and barbituric acid didnot change the band gap of g-C₃N₄, in contrast to DFT predictions,likely due to the low dopant level (C/N mass ratio of 0.61 vs 0.59).Nevertheless, the MCB_(0.07) composition showed noticeable band-tailabsorption in the visible light region (>460 nm) compared to undopedg-C₃N₄ compositions (U, M, and MC in FIG. 29), likely due to thepresence of midgap states, i.e., the electronic states located withinthe band gap. The midgap states can accommodate photoexcited electronsfrom the valence band, resulting in the absorption of photons withenergies smaller than the band gap for photocatalytic reactions. Inaddition to intrinsic photon absorption as reflected by the band gap,the morphology, structure, and particle size of g-C₃N₄ in reactionsuspension can also impact the photon absorption due to light reflectionand scattering on materials. The U composition had the highestabsorption of visible light (λ>400 nm), followed by the other samples(i.e., M, MC, and MCB_(0.07) compositions) with similar opticalabsorption, as illustrated in FIG. 30. Increased photon absorption ofthe U composition may improve the photocatalytic activity forcontaminant degradation.

Improved charge separation may enhance the photocatalytic activity ofg-C₃N₄ because of an increased amount of charge carriers forphotocatalytic reactions. PL intensity, representative of radiativecharge recombination, was used as an indirect evidence to characterizecharge separation. The PL intensity of U, MC, and MCB_(0.07) was similarand significantly lower than that of M (FIG. 31), and it suggests thatcharge recombination may be the most pronounced in M, and itsphotocatalytic performance may be limited. The PL peak of U, MC, andMCB_(0.07) also red-shifted compared to that of M (FIG. 31). Thereduction of PL intensity and red-shifted peaks indicate that theaddition of cyanuric acid and barbituric acid for the preparation ofsupramolecule-based g-C₃N₄ may lower the charge recombination comparedto M derived from melamine only. Holes are generally less mobile thanelectrons (i.e., diffusion length of several nanometers vs micrometers),and increased surface area and porosity in U, MC, and MCB_(0.07) couldfacilitate the migration of holes to the surface and subsequentreactions with contaminants rather than recombination with electrons.Carbon dopants may also facilitate hole localization and improve thereaction between charge carriers and contaminants, as suggested by DFTsimulations. Nevertheless, the PL only qualitatively characterizes theextent of radiative charge recombination but not nonradiative chargerecombination (dissipation as heat rather than photon emission), and itmay not necessarily explain the photocatalytic activity. For example,the PL intensity of U and MC was lower compared to that of MCB_(0.07),which suggested that U and MC might exhibit an increased chargeseparation. In contrast, the photocatalytic activity of MCB_(0.07) wasequal to or higher than that of U and MC for contaminant degradation.

It will be apparent that various other modifications and adaptations ofthe embodiments or application will be apparent to the person skilled inthe art after reading the foregoing disclosure without departing fromthe scope of the application and it is intended that all suchmodifications and adaptations come within the scope of the appendedclaims.

What is claimed is:
 1. A method of forming a supramolecule-based,carbon-doped graphitic carbon nitride (g-C₃N₄) composition, the methodcomprising: forming a suspension comprising an organic solvent and aprecursor mixture comprising melamine, cyanuric acid, and barbituricacid; drying the suspension at a first temperature to remove the organicsolvent and form a supramolecular complex; heating the supramolecularcomplex at a second temperature to form a carbon-doped g-C₃N₄composition.
 2. The method of claim 1, wherein the precursor mixturecomprises about 50 wt % melamine, about 2.5-49.75 wt % cyanuric acid,and about 0.25-47.5 wt % barbituric acid.
 3. The method of claim 1,wherein the precursor mixture comprises about 50 wt % melamine, about47.5-49.75 wt % cyanuric acid, and about 0.25-2.5 wt % barbituric acid.4. The method of claim 1, wherein the second temperature is betweenabout 450° C. and about 600° C.
 5. An article having a surface coatedwith a carbon-doped g-C₃N₄ composition formed according to claim
 1. 6.The article of claim 5, the article being any one of a kitchencounter-top, a cutting board, a hand-gripping portion of an appliance, ahand-gripping portion of a cabinet, a hand-gripping portion of a drawer,a desk, a table, a light fixture, a light switch, a food package, and afood processing surface.
 7. A water treatment system comprising acarbon-doped g-C₃N₄ composition formed according to claim
 1. 8. Thewater treatment system of claim 7, wherein the carbon-doped g-C₃N₄composition photocatalytically degrades persistent organicmicropollutants comprising pharmaceuticals and personal care products(PPCPs), endocrine disrupting compounds (EDCs), pesticides, andherbicides in the presence of ultraviolet (UV) or visible light.
 9. Themethod of claim 1, wherein the organic solvent is selected from thegroup consisting of chlorinated solvents, ethers, aldehydes and ketones.10. The method of claim 1, wherein the organic solvent is an alcohol.11. The method of claim 10, wherein the alcohol is ethanol.